1. Field of the Invention
The present invention pertains to double-seamed can ends and to loose metal can ends for double-seamed assembly on the open end of a can body such as a lightweight or thin-walled metallic or laminated, fiber-type composite can body which is relatively susceptible to perforation, penetration or puncture induced by radially inward buckling, pinlipping or pinwrinkling, of the hook end of the closure seaming wall against the can body during seaming or crimping of the loose metal end into assembly with the can body.
2. Description of the Prior Art
In the past, it has been conventional practice in the packaging field to manufacture metallic cans for the storage of comestibles, beverages and other various products. Customarily, such cans have comprised a tubular open-ended can body fabricated from metal sheet material having a thickness of about 0.008 inch and greater. Also, such can bodies commonly have been provided with an end closure which is double-seamed onto an outwardly flanged end portion provided at one or both opposite ends of the can body. Such double-seaming has been a favored form of assembling can end closures since it is readily adaptable to production line manufacturing capabilities and at the same time provides a reliable hermetic seal capable of withstanding substantial pressure differentials which may exist, or be encountered, between the ambient atmosphere and the interior confines of the filled container either during processing of the container's contents or during subsequent shipment and storage.
Due in large part to material, manufacturing and shipping cost considerations, a continually growing effort has evolved in the packaging field to provide lighter weight and less expensive cans for packaging an ever increasing variety of substances. In so far as metallic cans are concerned, this effort has evolved the emergence of thin-walled can bodies fabricated from light gauge metals such as aluminum and tin-plate. Moreover, such lightweight and thin-walled metallic cans now have attained substantial acceptance for packaging substances which require the development of internal pressures within the can such as, for example, are encountered in canning processes requiring sterilization of the can's contents or for packaging pressurized substances such as carbonated beverages and pressurized liquid spray materials. This trend towards less expensive and lighter weight cans has also engendered an alternative development and expanding usage of can bodies constructed from other various lightweight and relatively inexpensive materials such as plastic cans and laminated fiber-type cans commonly referred to as "composite cans". Such composite cans ordinarily include a tubular body portion fabricated from coated or uncoated fibrous materials, or composite interlayers of fibrous materials together with other compatible interlayer materials such as plastic and/or foil liners which assist in providing suitable gaseous or liquid impermeability, or in providing other physical characteristics dictated by the nature of the particular substance to be packaged and stored within the composite can.
As with the predecessor heavier gauge metal can bodies, it is still highly desirable to employ double-seamed end closures with thin-walled, lightweight metallic, plastic and composite can bodies to provide a reliable hermetic seal which is protective of the can's contents. This is especially desirable with respect to packaging comestibles, perishable products and most certainly carbonated beverages, pressurized liquids, and products which are heat-sterilized subsequent to packaging. However, the employment of double-seamed end closures with thin-walled, lightweight metallic, or plastic, or composite cans has been found to present a problem which was either insignificant or generally non-existant with heavier gauge metallic can bodies. In this regard, there is a common and natural tendency for the inwardly bent end, or hook end, portion of a double-seamed, metal can end to buckle radially inward and form sharp-edged, projections, or pointed barbs, at various random peripheral locations as the hook end is being peripherally constricted in size in consequence of being seamed, or crimped, into inter-locked, hermetic assembly with the can body. Due to the reduced wall thickness of lightweight metallic can bodies and the relative softness of plastic and composite can bodies, as compared with former heavier gauge metallic can bodies, the sharp-edged or pointed projections resulting from such buckling, commonly referred to in the art as "pinwrinkling" or "pinlipping", often partially penetrate or completely puncture the body portion of a lightweight metallic, or plastic, or composite can and thereby render it unsafe or unsuitable for packaging use. As a consequence, it has become highly desirable to attempt to obviate pinwrinkling or pinlipping as a source of can damage during the seaming operation. This desirability is particularly significant, since such can damage ordinarily occurs as one of the last stages of the can assembly and frequently after the can has been filled with its intended contents. Thus, the labor and material costs involved in filling and assembling the can are lost and not subject to recovery.